This invention relates to injection molding and more particularly to a process of producing molded parts of uniform density under varying material density conditions with a fast injection molding machine in which the ram bottoms at the conclusion of an injection stroke lasting approximately 1.5 seconds or less.
"Fast injecting molding" differs from normal injection molding in two distinct respects. First, the time interval necessary to complete the injection stroke, during which time the charge of material is injected into the mold cavity by a ram, is on the order of approximately 1.5 seconds or less, with injection intervals of 0.5 seconds or less being common. This is in contrast to a normal injection molding process in which the stroke interval to accomplish injection of the charge into the mold cavity typically takes 3-30 seconds or more. Second, in fast injection molding, due to ram inertia and the high speed achieved by the ram during a relatively short injection stroke interval, the ram bottoms out at the conclusion of each stroke, i.e., the ram tip reaches a forward limit of travel beyond which further forward motion is precluded by mechanical stops or limits, concluding ram advancement and terminating the injection stroke.
In molding parts of thermoplastic, thermoset, powdered metals and/or powdered ceramics, whether using conventional molding techniques or fast injection molding, it is always desirable to achieve uniformity in part weight from cycle-to-cycle. This is often difficult particularly when the density of the material being injected is varying in an unpredictable manner by reason of the evaporation of additives, uncontrolled variations in material temperature, ratio of regrind to virgin material and composition, and the like. Heretofore, a variety of approaches have been utilized to achieve uniform part density in molding processes of the conventional type (vis-a-vis fast injection molding). Unfortunately, due to the drastically different nature of fast injection molding, particularly the bottoming of the ram at the conclusion of the injection cycle, which typically takes approximately 1.5 seconds or less, the techniques found successful in conventional injection molding are incapable of providing uniform part density in fast injection molding.
Uniform part weight, an important consideration in conventional molding, is vastly more critical and important in fast injection molding. In fast injection molding, if insufficient material is injected into the mold when the ram bottoms and the injected material cools and shrinks, sink marks or voids in the molded product will occur. This is because the bottomed ram is incapable of supplementing the initially injected charge with additional material to compensate for part skrinkage as cooling occurs. If too much material is injected into the mold cavity, the mold will flash, and the flash will have to be trimmed from the molded part leaving undesirable surface imperfections where the flash were removed. These underpacking and overpacking conditions also affect the molded part weight and shrinkage, leading to dimensional instability and waste of material.